Riding and walk-behind mowers with drive system interlocks are known. Typically, the interlock systems prevent operation of the mower unless certain conditions have been satisfied. Examples include whether an operator is seated on the riding mower, whether the mower deck power take off is disengaged, or whether the drive system is in neutral.
One class of riding and walk-behind mowers provides zero radius turning. Such mowers typically include two independently driven wheels that allow the mower to turn by pivoting about a vertical axis located between the driven wheels because the wheels can be driven in opposite directions at the same time. Some zero turning radius mowers employ two control levers, each of the control levers controlling the movement of each of the driven wheels. Examples of such riding mowers are the TORO.RTM. Z MASTER.TM. zero radius tractors.
In such a design, the operator is freed from the need to shift the drive system between forward, reverse or neutral in a separate action from steering the mower because both steering and shifting are controlled by the control levers. For example, to go forward in a straight line, the operator need only push both levers forward. Similarly, to go in reverse, the operator merely pulled both levers backward. If no force is applied to the levers, the drive system is in neutral.
The real benefit to such a control system, however, is realized during the sharp maneuvering often required when performing tight trimming operations around shrubs and other objects. Because the drive wheels can be driven in forward or reverse by changing the direction of the control levers, the operator is not required to stop, use a clutch or shift while also steering the mower.
Because these riding mowers also include interlock systems, it is necessary to determine whether the drive systems were in neutral. Typically, the monitoring the drive systems involves the use of electromechanical plunger switches in which the control levers depress a plunger to close a switch when the control levers are in the position that placed the drive wheels in neutral. Although plunger switches may typically operate with sufficient reliability in many situations, their useful life can be problematic when used in conjunction with the control levers of riding mowers including a drive motor for each driven wheel.
The reliability issue is caused in large part by the frequent cycling of each drive motor between forward and reverse. With each change between forward and reverse, the neutral plunger switch is depressed briefly by the control lever associated with that wheel as it passes through the neutral position. When the riding mowers are used to mow areas in which maneuverability is required, i.e., the wheels are repeatedly being driven in forward and reverse, the plunger switches can be cycled many times.
Another potential problem is that, because the control levers are mounted to pivot during movement between forward and reverse, they typically pass the plunger switch along a path that is generally transverse to the axis of activation of the plunger switch, i.e., the axis along which the plunger switch is designed to move during operation. As a result, the plunger switch is subjected to an off-axis force that may reduce its useful life as compared to applications in which the plunger switch is depressed along its axis of activation.
The above problems are exacerbated due to the generally dirty environment in which the plunger switches operate. The switches can quickly become covered with dirt, grass clippings, water, oil, grease etc. That contamination can cause the seals in the plunger switch to fail early, thereby necessitating replacement of the switch.